The invention relates to a twin plasma torch apparatus.
In a twin plasma torch apparatus, the two torches are oppositely charged i.e. one has an anode electrode and the other a cathode electrode. In such apparatus, the arcs generated by each electrode are coupled together in a coupling zone remote from the two torches. Plasma gases are passed through each torch and are ionised to form a plasma which concentrates in the coupling zone, away from torch interference. Material to be heated/melted may be directed into this coupling zone wherein the thermal energy in the plasma is transferred to the material. Twin plasma processing can occur in open or confined processing zones.
Twin plasma apparatus are often used in furnace applications and have been the subject of previous patent applications, for example EP0398699 and U.S. Pat. No. 5,256,855.
The twin arc process is energy efficient because as the resistance of the coupling between the two arcs increases remote from the two torches, the energy is increased but torch losses remain constant. The process is also advantageous in that relatively high temperatures are readily reached and maintained. This is attributable to both the fact that the energy from the two torches is combined and also because of the above mentioned efficiency.
However, such processes have disadvantages. If the plasma torches are in close proximity to one another and/or are enclosed within a small space, there is a tendency for the arcs to destabilise, particularly at higher voltages. This side-arcing occurs when the arcs preferentially attach themselves to lower resistance paths.
The problem of side-arcing in current twin torch apparatus has lead to the development of open processing units in which the plasma torches are substantially spaced apart, with low resistance paths removed from vicinity, as described in U.S. Pat. No. 5,104,432. In such units, the process gas is free to expand in all directions in these applications. However, such arrangements are not suitable for all processing applications, particularly when expansion of process gases needs to be controlled e.g. production of ultra fine powders.
In current systems with confined processing zones, the torch nozzles project into the chamber so that the chamber walls, which have a low resistance, are removed from the vicinity of the plasma arc. This awkward construction inhibits side-arcing and encourages coupling of the arcs. However, the protruding nozzles provide surfaces on which melted material may precipitate. This not only results in wastage of material but shortens the life of the torches.
The present invention provides a twin plasma torch assembly comprising:
(a) at least two twin plasma torch assemblies of opposite polarity supported in a housing, said assemblies being spaced apart from one another and each comprising
(i) a first electrode,
(ii) a second electrode which is or is adapted to be spaced apart from the first electrode by a distance sufficient to achieve a plasma arc therebetween in a processing zone;
(b) means for introducing a plasma gas into the processing zone between the first and second electrodes;
(c) means for introducing shroud gas to surround the plasma gas;
(d) means for supplying feed material into the processing zone; and
(e) means for generating a plasma arc in the processing zone.
The shroud gas confines the plasma gas, inhibits side-arcing, and increases plasma density. The invention therefore provides an assembly in which the torches are inhibited from side-arcing, and thus facilitates the miniaturisation of torch design where distance to low resistance paths are small. The use of shroud gas can also eliminate the need for torch nozzles to extend beyond the housing.
The shroud gas may be provided at various locations along the electrodes, particularly in cylindrical torches where arcs are generated along the length of the electrodes. However, preferably, each torch has a distal end for the discharge of plasma gas and the means for supplying shroud gas provides shroud gas downstream of the distal end of each electrode. Therefore, reactive gases such as oxygen may be added to the plasma without degrading the electrode. The practical applicability of plasma torches is increased by the facility to add reactive gases downstream of the electrode.
In a preferred embodiment, each plasma torch comprises a housing which surrounds the electrode to define a shroud gas supply duct between the housing and the electrodes, wherein the end of the housing is tapered inwards towards the distal end of the torch to direct flow of the shroud gas around the plasma gas.
The twin plasma torch assembly of the present invention may be used in an arc reactor having a chamber to carry out a plasma evaporation process to produce ultra-fine (i.e. sub-micron or nano-sized) powders, for example aluminium powders. The reactor may also be used in a spherodisation process.
The chamber will typically have an elongate or tubular form with a plurality of orifices in a wall portion thereof, a twin plasma torch assembly being mounted over each orifice. The orifices, and thus the twin plasma torch assemblies, may be provided along and/or around said tubular portion. The orifices are preferably provided at substantially regular intervals.
The distal ends of the first and/or second electrodes, for the discharge of plasma gas will typically be formed from a metallic material, but may also be formed from graphite.
The plasma arc reactor preferably further comprises cooling means for cooling and condensing material which has been vaporised in the processing zone. The cooling means comprises a source of a cooling gas or a cooling ring.
The plasma arc reactor will typically further comprise a collection zone for collecting processed feed material. The process feed material will typically be in the form of a powder, liquid or gas.
The collection zone may be provided downstream of the cooling zone for collecting a powder of the condensed vaporised material. The collection zone may comprise a filter cloth which separates the powder particulate from the gas stream. The filter cloth is preferably mounted on an earthed cage to prevent electrostatic charge build up. The powder may then be collected from the filter cloth, preferably in a controlled atmosphere zone. The resulting powder product is preferably then sealed, in inert gas, in a container at a pressure above atmospheric pressure.
The plasma arc reactor may further comprise means to transport processed feed material to the collection zone. Such means may be provided by a flow of fluid, such as, for example, an inert gas, through the chamber, wherein, in use, processed feed material is entrained in the fluid flow and is thereby transported to the collection zone.
The means for generating a plasma arc in the space between the first and second electrodes will generally comprise a DC or AC power source.
The apparatus according to the present invention may operate without using any water-cooled elements inside the plasma reactor and allows replenishment of feed material without stopping the reactor.
The means for supplying feed material into the processing zone may be achieved by providing a material feed tube which is integrated with the chamber and/or the twin torch assembly. The material may be particulate matter such as a metal or may be a gas such as air, oxygen or hydrogen or steam to increase the power at which the torch assembly operates.
Advantageously, the distal ends of first and second electrodes, for the discharge of plasma gas, do not project into the chamber.
The small size of the compact twin torch arrangement according to the present invention allows many units to be installed onto a product transfer tube. This enables easy scale-up to typically over 10 times to give a full production unit without scale up uncertainty.
The present invention also provides a process for producing a powder from a feed material, which process comprises:
(A) providing a plasma arc reactor as herein defined;
(B) introducing a plasma gas into the processing zones between the first and second electrodes;
(C) generating a plasma arc in the processing zones between the first and second electrodes;
(D) supplying feed material into the plasma arcs, whereby the feed material is vaporised;
(E) cooling the vaporised material to condense a powder; and
(F) collecting the powder.
The feed material will generally comprise or consist of a metal, for example aluminium or an alloy thereof. However, liquid and/or gaseous feed materials can also be used. In the case of a solid feed, the material may be provided in any suitable form which allows it to be fed into the space between the electrodes, i.e, into the processing zone. For example, the material may be in the form of a wire, fibres and/or a particulate.
The plasma gas will generally comprise or consist of an inert gas, for example helium and/or argon.
The plasma gas is advantageously injected into the space between the first and second electrodes, i.e. the processing zone.
At least some cooling of the vaporised material may be achieved using an inert gas stream, for example argon and/or helium. Alternatively, or in combination with the use of an inert gas, a reactive gas stream may be used. The use of a reactive gas enables oxide and nitride powders to be produced. For example, using air to cool the vaporised material can result in the production of oxide powders, such as aluminium oxide powders. Similarly, using a reactive gas comprising, for example, ammonia can result in the production of nitride powders, such as aluminium nitride powders. The cooling gas may be recycled via a water-cooled conditioning chamber.
The surface of the powder may be oxidised using a passivating gas stream. This is particularly advantageous when the material is a reactive metal, such as aluminium or is aluminium-based. The passivating gas may comprise an oxygen-containing gas.
It will be appreciated that the processing conditions, such as material and gas feed rates, temperature and pressure, will need to be tailored to the particular material to be processed and the desired size of the particles in the final powder.
It is generally preferable to pre-heat the reactor before vaporising the solid feed material. The reactor may be preheated to a temperature of at least about 2000xc2x0 C. and typically approximately 2200xc2x0 C. Pre-heating may be achieved using a plasma arc.
The rate at which the solid feed material is fed into the channel in the first electrode will affect the product yield and powder size.
For an aluminium feed material, the process according to the present invention may be used to produce a powdered material having a composition based on a mixture of aluminium metal and aluminium oxide. This is thought to arise with the oxygen addition made to the material during processing under low temperature oxidation conditions.